Steam cracking, also referred to as pyrolysis, has long been used to crack hydrocarbon feedstocks into a wide range of relatively high value molecules including ethylene, propene, butenes, steam cracked gas oil (“SCGO”), etc. Besides these useful products, hydrocarbon pyrolysis can also produce a significant amount of relatively low-value heavy products, such as pyrolysis tar. When the pyrolysis is produced by steam cracking, the pyrolysis tar is identified as steam-cracker tar (“SCT”). Economic viability of refining processes relies in part on the ability to incorporate as much of the product and residual fractions, such as SCT into the value chain. One use of residual and/or relatively low value products is to blend these fractions with other hydrocarbons, e.g., with other feedstreams or products.
It is conventional to subject the SCT to hydroprocessing in the presence of a utility fluid, e.g., a solvent having significant aromatics content. The hydroprocessed SCT product generally has a decreased viscosity, decreased atmospheric boiling point range, and increased hydrogen content over that of the SCT feed, resulting in improved compatibility with fuel oil blend-stocks. Additionally, hydroprocessing the SCT in the presence of utility fluid produces fewer undesirable byproducts and the rate of increase in reactor pressure drop is lessened. Conventional processes for SCT hydroprocessing is disclosed in U.S. Pat. Nos. 2,382,260 and 5,158,668; and in International Patent Application Publication No. WO 2013/033590, which involves recycling a portion of the hydroprocessed tar for use as the utility fluid.
SCT, however, generally contains an undesirable amount of particulate matter, e.g., coke particles. The particulate matter represents about 0.5 wt. % of the SCT and ranges in size from 1 to about 1000 μm. Depending on the size of a commercial-scale SCT processor, this can represent approximately 5-10 tons of solids per day being sent through the hydroprocessing unit. The presence of these particulates generally leads to a rapid increase in reactor pressure drop and catalyst deactivation, resulting from the formation of undesirable foulant deposits on the catalyst, reactor internals, and ancillary equipment. As the amount of these deposits increases, the yield of the desired upgraded pyrolysis tar (upgraded SCT) decreases and the yield of undesirable byproducts increases. Moreover, the hydroprocessing reactor's pressure drop rapidly increases to a point where the reactor is inoperable.
Removing particulate matter, especially particulate matter that may be a foulant or foulant precursor, is not trivial. Conventional filtration techniques have difficulty handling the load of solids and require frequent removal of the retentate to remain operable, resulting, it is believed, from relatively small particles in the SCT which very effectively clog the filter mechanism. Removing particulates from SCT by conventional sedimentation, where particles in the SCT segregate to the bottom of a large pool, requires commercially-impractical residence times.
Thus, a method for reducing the impact of particulate matter in SCT and/or rendering the SCT more amenable to hydroprocessing would be beneficial.